Three-dimensional printing, also known as additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Many three-dimensional printing technologies use an additive process in which an additive manufacturing device forms successive layers of the part on top of previously deposited layers. Some of these technologies use extruder heads that soften or melt extrusion material, such as ABS plastic, into thermoplastic material and then extrude the thermoplastic material in a predetermined pattern. The printer typically operates the extruder head to form successive layers of the thermoplastic material that form a three-dimensional printed object with a variety of shapes and structures. After each layer of the three-dimensional printed object is formed, the thermoplastic material cools and hardens to bond the layer to an underlying layer of the three-dimensional printed object. This additive manufacturing method is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
These additive manufacturing devices can produce highly functional three-dimensional (3D) parts, but typically the time of manufacture can be quite lengthy. One parameter that affects the time for object manufacture is the viscosity of the thermoplastic material extruded from the extruder head or heads. Lowering the viscosity of the thermoplastic material by increasing the extruder temperature can reduce time for the production process, but lowered viscosity can have a detrimental effect on the ability of the extruder to form detailed features in the part. One problem with changing the temperature of the extruded thermoplastic material arises because thermoplastic material continues to exude from the extruder while the temperature of the extruder is changed. This thermoplastic material that has a viscosity in a transition range to the next desired viscosity can produce defects in the object being formed. What is needed is a three-dimensional object printer that enables control of the viscosity of the thermoplastic material being extruded into a part.